1. Field of the Invention
The present invention relates to a focus detection device using a pupil division phase-difference focus detection method.
2. Description of the Related Art
In a focus detection device which is install in a single-lens reflex (SLR) camera, in the case where a pupil division phase-difference focus detection method is employed, a pair of object images obtained by pupil division are projected onto a pair of areas on a line sensor, and a defocus amount is obtained from a phase difference between the pair of object images on the line sensor. Specifically, when an interval between the pair of object images formed on the line sensor is a predetermined length, focusing is attained; when the interval is smaller than the predetermined length, it is judged as a front focus; and when the interval is larger than the predetermined length, it is judged as a rear focus. The amount of defocusing from an in-focus position is output as a defocus amount.
In regard to pupil division phase-difference focus detection methods, vertical line detection, horizontal line detection, and cross-type detection are known in the art. The vertical line detection method detects a focus state of an object that has a contrast component in a horizontal direction. The horizontal line detection method detects a focus state of an object that has a contrast component in a vertical direction. In the cross-type detection method, both the vertical line detection and the horizontal line detection are performed.
For example, Japanese Examined Patent Publication No. H07-74855 teaches a cross-type focus detection method in which a phase difference is obtained from a value (synthesized sensor output) obtained by the sum of the outputs of vertical and horizontal line sensor arrays, whereby enhancing the focus detection accuracy. In Japanese Unexamined Patent Publication No. 2005-300844, two line sensor arrays using a cross-type focus detection method are arranged adjacently in parallel to each other while being arranged so that the line sensors are relatively shifted (mutually deviated), whereby the pixel pitch can be reduced without reducing sensitivity; consequently, the focus detection accuracy is enhanced.
However, even if such related-art vertical line detection, horizontal line detection, and cross-type detection methods are performed, when an object has a contrast component (a luminance distribution) in an oblique direction, a focal shift still occurs. Japanese Examined Patent Publication No. H07-74855 and Japanese Unexamined Patent Publication No. 2005-300844 do not describe a focus detection method for dealing with an object that has a contrast component in an oblique direction, and there is no disclosure whatsoever of an oblique line detection method which detects a focus state of an object that has a contrast component in the oblique direction.
It is possible to implement an oblique line detection method by utilizing an output of a line sensor for vertical line detection or a line sensor for horizontal line detection. However, if the positions of object images formed on such (vertical or horizontal line detection) line sensors are deviated due to part accuracy or assembly error, focus detection is applied to different areas from those of the object images, and thus the focus detection accuracy is significantly and undesirably reduced. In order to suppress the reduction of the accuracy of such an oblique line detection method due to part accuracy, Japanese Unexamined Patent Publication No. H02-272410, for example, proposes an arrangement of line sensors that is determined in accordance with error in a separator lens thereof. However, a line sensor is required to be produced to correspond to the cavity of the separator lenses, which are resin molded, leading to an increase in cost. In Japanese. Unexamined Patent Publication No. 2002-174766, the precision of the separator lens itself is increased to thereby enhance the precision of the oblique line detection. However, there are limits to the machining precision of the parts/components (i.e., part accuracy), and in addition, the influence of assembly error on an optical system which occurs when a line sensor is assembled cannot be corrected.